The present invention relates to an apparatus for exciting a step motor, specifically, a step motor which is employed as a drive source for conveying document originals in a reading device such as may typically be used in a facsimile system.
An original reading device of this type, which operates using a planar scanning technique, generally employs a step motor as an original conveying drive source. In such an original reading device, the feed pitch (the distance of movement in the auxiliary scanning direction as defined by the stroke length of the step motor) of one line of the original can be suitably changed so as to obtain a video signal corresponding to the image of the original which is equivalent to equal size, enlarged size or reduced size magnification settings. In order to change the original feed pitch, in one prior art approach a power transmission speed reduction mechanism is provided between the step motor and the original conveying mechanism. In this conventional system, a plurality of different speed reduction ratios are provided with the power transmission speed reduction mechanism so that the amount of movement per stroke of the step motor transmitted to the original conveying mechanism can be set by the speed reduction ratio selected by a switching device. However, this system is disadvantageous in that the magnification switching mechanism is intricate and the original reading mechanism is bulky.
In order to eliminate these difficulties, a system has been proposed in which the angle of rotation of the rotor per exciting pulse is changed. In that system, the auxiliary scanning distance (the distance between adjacent scanning lines on the original) is varied by changing the number of input pulses to the step motor, for instance, four input pulse signals for equal size magnification, three input pulse signals for enlarging magnification, and five input pulse signals for reducing magnification. In this case, the transmission speed reduction system provides a speed reduction ratio of 4:1. Unfortunately, the angular speed of the rotor is unavoidably increased because the number of input pulse signals per unit of time must be increased to maintain a desired reading speed. However, upon increasing the angular speed of the rotor, it becomes difficult to produce sufficient torque for every step of the rotor. Accordingly, it is difficult to employ such a step motor in an original reading device. In order to be effective, it is essential for the rotor to provide a relatively small angular speed so that sufficient torque is produced for every application of the input pulse signal. Accordingly, it is required that an exciting system be provided which reduces the step angle of the rotor per input pulse signal (hereinafter referred to as "a unitary step angle" when applicable).
Step motor exciting systems in which the unitary step angle of the rotor is reduced include a proposed so-called "exciting current unbalance type W1-2 phase exciting system". In this sytem, exciting currents supplied to the exciting coils of the step motor driven in 1-2 phase excitation are suitably switched between two values which are in a predetermined ratio, whereby the unitary step angle is reduced.
FIG. 1 is a waveform diagram for a description of the stepping operation of a rotor in which the W1-2 phase exciting system is employed. FIG. 1(a) shows the position of the stator, and FIGS. 1(b) through 1(c) show the positions of the rotor with respect to the stator. When a current of one ampere, for instance, is supplied to a first phase exciting coil (not shown and hereinafter referred to as "an A-phase exciting coil" when applicable), the pole piece 1A of the stator on which the A-phase exciting coil is wound is excited. At the same time, as shown in FIG. 1(b), the rotor 2 is stepped to a position where the pole piece 2A of the rotor directly confronts the pole piece 1A of the stator. When, under this condition, a current of 0.4 ampere, for instance, is supplied to the second phase exciting coil (not shown and hereinafter referred to as "a B-phase exciting coil" when applicable), the pole piece 1B of the stator on which the B-phase exciting coil is wound is excited.
The currents supplied to the A-phase and B-phase exciting coils are different in value from each other, and therefore the magnetic energy produced in the pole piece 1A of the stator is different from that produced in the pole piece 1B. Because of this, the rotor 2 steps until the pole piece 2A comes to a predetermined distance from the pole piece 1A between the pole pieces 1A and 1B, nearer the pole piece 1A than the pole piece 1B, as indicated in FIG. 1(c). When currents of one ampere are supplied to the A-phase and B-phase exciting coils, the magnetic energy produced in the pole piece 1A is equal to that produced in the pole piece 1B. In that case, the rotor 2 steps until the pole piece 2A reaches the midpoint between the pole pieces 1A and 1B, as shown in FIG. 1(d). When, under this condition, the current supplied to the A-phase exciting coil is changed from one ampere to 0.4 ampere, the magnetic energy in the pole piece 1A becomes different from that in the pole piece 1B. Accordingly, the rotor 2 steps until the pole piece 2A comes to a predetermined distance from the pole piece 1B between the pole pieces 1A and 1B, nearer the pole piece 1B than the pole piece 1A. As is clear from the above description, when the rotor steps four times successively, an operation of rotation for one step angle, corresponding to the unitary step angle in the single-phase exciting system, is effected.
FIG. 2 is a graph plotting the angular position of the rotor with time under no load with the step motor being driven in accordance with the W1-2 phase exciting system. In FIG. 2, t designates time, and .theta. the angle of rotation of the rotor.
Under the condition that no load is applied, the rotor repeats stepping and stopping quickly whenever an input pulse signal is applied thereto, thus exhibiting the regular staircase characteristic curve shown in FIG. 2. On the other hand, if the step motor is employed in a conveying system in which a relatively large load is imposed on the motor, the characteristic curve of rotation becomes as shown in FIG. 3. In this case, the torque produced is not large, and the rotor steps relatively slowly. In the W1-2 phase exciting system, the current supplied to each exciting coil is changed in two steps, and therefore four exciting phases are provided. In the general case, as the number of steps in which the exciting current is changed is reduced, the magnetic energy produced for each exciting phase, and therefore the torque produced during the stepping of the rotor is not uniform among all steps. Accordingly, the stepping speed of the rotor fluctuates as illustrated.
Assuming that the amount of movement of an original which is provided when the rotor is stepped four times successively is equal to an auxiliary scanning width (scanning line spacing) and one cycle of four exciting states as described above is completed every auxiliary scanning for each line of the original, the reading scanning density is uniform for every line, as shown in FIG. 4(a). In this operation, a video signal accurately representing the character "A" is obtained. However, if the movement of the original is not uniform for the reasons discussed above, the reading scanning density will not be uniform among all lines. In this case, a correct video signal cannot be obtained for the character "A".
In view of the foregoing, an object of the present invention is to provide a step motor exciting system in which uniform torque is produced for every step of the rotor, even when the step motor is under load.